Airborne particulate matter (PM) is a growing concern worldwide and is known to have an adverse impact notably on human health, on the environment and on climate change. Many large cities around the world go through frequent and/or extended periods of time during which PM concentration levels exceed accepted thresholds. In 2014, the World Health Organization estimated that ambient air pollution contributes to 6.7% of all deaths worldwide. Studies have also established a link between air pollution and strokes from correlations with PM measurements performed in large cities, while other studies have associated air pollution with autism and learning disabilities in young children. Airborne PM can have different origins and chemical compositions, and can travel over long distances, so that regulations aimed at managing them are getting more stringent and complex. Recent trends and advances in environmental monitoring have also lead to new demands in terms of PM control and management.
Particle sizing techniques based on light scattering are known and have been used in different fields and with different types of materials. Such techniques generally involve providing a sample of particles, illuminating the sample, measuring light scattered by the particles, and analyzing the scattering measurements to obtain particle size information. Particle size distributions can also be obtained through statistical data accumulated over time on individual particles or by using inversion methods applied to a representative population of the sample.
Several commercially available systems use light scattering for determining the size of particles, typically using a laser diode for producing the light beam for particle illumination. The particles are usually supplied to a chamber by a vacuum-based pumping system that samples part of the ambient medium. The size of the chamber is typically small, with sidewalls of the order of a few millimeters (mm) long. The light beam usually provides a uniform illumination of the particles to reduce measurement errors arising from the fact that different scattered signals may originate from particles illuminated by different portions of the beam. An optical detector measures the amount of light scattered from the particles, usually at a scattering angle of about 90° relative to the propagation direction of the illumination light beam. Such a “sideway” scattering detection scheme may allow the particle sizing system to be made more compact and the amount of stray light reaching the detector to be reduced.
It is known that the particle sizing systems discussed above have some drawbacks and limitations. First, the intensity of light scattered at 90° is generally quite sensitive to the composition of the particles. As a result, proper calibration of the particle sizing system as a function of particle composition is generally unavoidable to ensure meaningful particle size measurements. In addition, the vacuum-based pumping systems typically used with conventional particle sizing systems are susceptible to mechanical wear and damage. As a result, these pumping systems generally require careful inspection and maintenance which, in turn, can substantially increase the operating costs of the particle sizing systems. Pumping systems also typically have to be calibrated to ensure that the supply rate of particles to the chamber is known, since its value will affect the number of particles analyzed per unit of time. Yet another limitation of conventional systems comes from the fact that particles are sampled from the ambient medium and supplied to the chamber by the pumping system. The sampling process can cause different measurement errors and biases due, for example, to inlets being biased to a certain particle size, to particles breaking up as a result of hitting system components, to particle deposition on wall surfaces, and the like.
Other types of particle sizing systems have been developed where forward rather than sideway scattered light is detected. In such systems, the intensity of the detected scattered signals may, in principle, be made less sensitive to particle composition. However, these systems generally rely on inversion methods such as mentioned above, which yield particle size distributions rather than individual particle sizes and often necessitate a model or a priori knowledge of the particle composition to be applied.
Accordingly, many challenges remain in the development of particle sizing systems and methods that use light scattering for determining the size of individual particles in a sample, while also being less sensitive to particle composition and involving less mechanical maintenance.